<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>css3过渡效果-进度条案例</title>
    <style>
        .bar{
            width: 150px;
            height: 15px;
            border: 1px solid red;
            /* 圆矩形：盒子高度的一半 */
            border-radius: 7px;
            padding: 1px;
        }
        .bar_in{
            width: 50%;
            height: 100%;
            background-color: red;
            border-radius: 7px;
            /* 谁做过渡，谁加 */
            transition: width 1s;
            
        }
        .bar_in.bar_in:hover{
            width: 100%;
        }
    </style>
</head>
<body>
    <div class="bar">
        <div class="bar_in"></div>
    </div>
    
</body>
</html>